Injection molded resin gear, injection molded resin rotating body, and injection molded article

ABSTRACT

An injection molded resin gear  1  has a rim  6  having teeth  7  on its outer periphery, a hub  4  for receiving therein a shaft, and a web  6  connecting the rim  6  to the hub  4 . The thickness (t 1 ) of the bottom  15  of the rim  6  is smaller than the thickness (t 2 ) of the web  5 . On the side faces  8  and  8  of the web  5 , annular circumferential ribs  10, 11  and  12  protruding concentrically with the rim  6  and hub  4  are formed. The relationship between the thickness (t 1 ) of the bottom  15  of the rim  6 , the thickness (t 2 ) of the web  5  and the thickness (t 3 ) of the circumferential rib  12  is t 1 ≦t 3 &lt;t 2.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to a gear of a resin, such as aninjection molded resin gear, for use in power transmission devices forvarious precision instruments, such as color copying machines, colorprinters and video tape recorders, various automotive parts andindustrial equipment. The invention also relates to an injection moldedresin rotating body, such as an injection molded resin gear, roller,pulley or sprocket, which may be often used in power transmissionportions. Moreover, the invention relates to an injection molded articlewhich is used as a dynamic injection molded resin rotating body, such asan injection molded resin gear or roller, or as a stationary shaftsupporting member.

2. Description of the Prior Art

For example, tandem type color copying machines and color printers aredesigned to sequentially carry out steps of forming images of black (K),magenta (M), cyanogen (C) and yellow (Y) in a direction, in which atransfer material, such as a copy paper, is carried, to sequentiallysuperpose and transfer images of different colors to the transfermaterial, which is carried by a carrier belt, to print a desired colorimage on the transfer material. In such color copying machines and colorprinters, an image forming unit for each color (K, M, C, Y) and acarrier belt are connected to a motor via gears to be operated by thedriving force of the motor. Therefore, in order to enable a high qualitycolor print, it is required to precisely operate the image forming unitfor each color (K, M, C, Y) and the carrier belt to precisely transfereach color image to the transfer material. For that reason, theprecision of gears for transmitting the rotation of the motor to theimage forming unit and carrier belt is important. That is, if theprecision of the gears is bad, the rotation of the motor is notprecisely transmitted to the image forming unit and carrier belt, sothat operation failure, such as irregular rotation, is caused in theimage forming unit and carrier belt, thereby causing print failure, suchas color deviation.

Users have requested to reduce the size, weight and price of the abovedescribed color copying machines and color printers and to reduceoperation noises thereof.

Therefore, in the above described color copying machines and colorprinters, an injection molded resin gear, which has a higher precisionthan that of a cut metal gear and which can more greatly reduce theweight and operation noises than those of the metal gear, has beenwidely used as a gear for transmitting the rotation of a motor to animage forming unit or a carrier belt.

In a process for forming such an injection molded resin gear, a moltenresin injected into a cavity shrinks when it is cooled to be solidified.The amount of shrinkage of the molten resin in a thick region of theresin is larger than that in a thin region thereof. As a result, forexample, in an injection molded resin gear 31 as shown in FIG. 10, thereare inherent problems (problems which are not caused by cut metal gears)in that the diameter of the connecting portion of a web 32 to a rim 33is more greatly reduced than that of a portion of the rim 33 on the endside thereof, so that a shrink mark 34 is produced in a substantiallycentral portion in face width directions to deteriorate the precision ofthe gear (e.g. total alignment error). Therefore, in order to furtherimprove the precision of the gear, it is important to reduce thedifference in shrinkage between parts.

As shown in FIG. 11, the inventors have proposed an injection moldedresin gear 31 wherein a web 32 has a plurality of circumferential ribs36, 37 and 38 arranged concentrically with a hub 35, at least one 37 ofthe plurality of circumferential ribs 36–38 being arranged so as to facea plurality of injection molding pin point gates 40 (see Japanese PatentLaid-Open No. 8-25501). This conventional injection molded resin gear 31is formed by allowing most of a molten resin, which is injected from thepin point gates 40, to flow along the circumferential rib 37 incircumferential directions and by allowing the molten resin to flowinwardly and outwardly in radial directions from the circumferential rib37. Thus, the flow of the resin is equalized in radial directions as awhole to make distribution in rate of shrinkage uniform, so that thegear is formed so as to have an excellent roundness. However, in recentyears, with the innovative improvement of performance of precisioninstruments and so forth using the injection molded resin gear 31, it isdesired to provide more precise injection molded resin gears.

Similar to the above described injection molded resin gear 31, it isdesired to develop a technique for more precisely forming the shape ofan injection molded resin rotating body, such as an injection moldedresin pulley, sprocket or roller, which forms a part of a powertransmission mechanism, to more precisely and smoothly transmit therotation thereof.

In precision instruments and so forth using the injection molded resingear 31, it is required to precisely support a dynamic (sliding orrotating) shaft or a stationary (fixed) shaft on a frame or the like. Insuch a case, there is used a shaft supporting member (an injectionmolded article) which is injection-molded so as to substantially havethe same shape as that of an injection molded resin roller. The outercylindrical portion of this shaft supporting member is designed to befixed to a frame or the like, and the inner cylindrical portion thereofis designed to support thereon a shaft. The shaft supporting member doesnot rotate unlike injection molded resin rotating bodies, such asinjection molded resin gears and rollers. However, it is desired toimprove the precision of the shape of the shaft supporting member inorder to prevent the deviation in alignment of the shaft and so forth.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to eliminate theaforementioned problems and to provide a precise injection molded resingear.

It is another object of the present invention to improve the precisionof an injection molded resin rotating body, such as an injection moldedresin pulley, sprocket or roller, which forms a power transmissionmechanism, similar to an injection molded resin gear.

It is a further object of the present invention to improve the precisionof an injection molded article, such as a shaft supporting member, whichis formed to substantially have the same shape as that of an injectionmolded resin roller so as to be capable of supporting thereon a movableor stationary shaft.

In order to accomplish the aforementioned and other objects, accordingto one aspect of the present invention, an injection molded resin gearcomprises: a rim having teeth on an outer periphery thereof; a hub forreceiving therein a shaft; and a web connecting the rim to the hub,wherein a bottom of the rim has a smaller thickness than that of theweb.

In this injection molded resin gear, the thickness of the bottom of therim may be smaller than the thickness of the web in the vicinity of aconnecting portion of the rim to the web.

The injection molded resin gear may further comprise an annularcircumferential rib formed on the web, the circumferential ribprotruding concentrically with the rim and hub, and the relationshipbetween the thickness (t1) of the bottom of the rim, the thickness (t2)of the web and a thickness (t3) of the circumferential rib may be t1≦t3<t2.

The injection molded resin gear may further comprise an annularcircumferential rib formed on the web nearer to the hub than the rim,the circumferential rib protruding concentrically with the rim and hub,and the injection molded resin gear may be formed by injecting a moltenresin from a plurality of pin point gates which are arranged at regularintervals in circumferential directions of the circumferential rib.

The injection molded resin gear may further comprise a resin guidingprotrusion for guiding a molten resin, which is injected from a pinpoint gate, to the hub, the resin guiding protrusion protruding from anouter periphery of the hub on one side of the web so as to correspond tothe pin point gate.

The injection molded resin gear may further comprise a plurality ofannular circumferential ribs formed on the web, the circumferential ribsprotruding concentrically with the rim and hub, at least adjacent two ofthe plurality of circumferential ribs being connected to each other by aplurality of radial ribs extending in radial directions, and therelationship between the thickness (t1) of the bottom of the rim, thethickness (t2) of the web and a thickness (t3) of at least one of thecircumferential ribs may be t1≦t3<t2.

The injection molded resin gear may further comprise a detent formed onthe web, the detent engaging a gear driving member, which rotates withthe shaft, to allow the injection molded resin gear to rotate with thegear driving member. This detent may be a radial rib.

According to another aspect of the present invention, an injectionmolded resin gear comprises: a rim; a hub; and a web connecting the rimto the hub, the web having a plurality of annular circumferential ribswhich surround the hub concentrically with each other, at least adjacenttwo of the circumferential ribs being connected to each other in radialdirections by means of a plurality of radial ribs which are arranged atregular intervals in circumferential directions, wherein the injectionmolded resin gear is formed by injecting a molten resin from a pluralityof pin point gates arranged at regular intervals on a circle, which isarranged concentrically with the hub, nearer to the hub than theadjacent two of the circumferential ribs, the radial ribs being formedso as to be displaced from a straight line which radially extends from arotation center of the gear via each of the pinpoint gates and so as tobe displaced from a straight line which passes through the rotationcenter of the gear and a center between adjacent two of the pin pointgates, and no radial rib is formed between one of the circumferentialribs, which is nearest to the pin point gates, and the pin point gates.

According to a further aspect of the present invention, an injectionmolded resin rotating body comprises: a substantially cylindrical rim; ahub for receiving therein a shaft; and a web connecting the rim to thehub, wherein a thinnest portion of the rim has a smaller thickness thanthat of the web.

According to a still further aspect of the present invention, aninjection molded article comprises: an outer cylindrical portion; aninner cylindrical portion for receiving therein a shaft; and adisk-shaped portion connecting the outer cylindrical portion to theinner cylindrical portion, wherein the outer cylindrical portion has asmaller thickness than that of the disk-shaped portion.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood more fully from the detaileddescription given herebelow and from the accompanying drawings of thepreferred embodiments of the invention. However, the drawings are notintended to imply limitation of the invention to a specific embodiment,but are for explanation and understanding only.

In the drawings:

FIG. 1 is a side view of the first preferred embodiment of an injectionmolded resin gear according to the present invention, which is viewedfrom the side of a web;

FIG. 2 is a sectional view taken along line II—II of FIG. 1;

FIG. 3 is an enlarged view of a part of a rim of the injection moldedresin gear of FIG. 1;

FIG. 4 is an enlarged sectional view of a part of FIG. 2;

FIG. 5 is a side view of a modified example of the injection moldedresin gear of FIG. 1;

FIG. 6 is a view showing the relationship between the injection moldedresin gear of FIG. 1 and a gear driving member;

FIG. 7 is a sectional view taken along line VII—VII of FIG. 6;

FIG. 8 is a side view of the second preferred embodiment of an injectionmolded resin gear according to the present invention, which is viewedfrom the side of a web;

FIG. 9 is a sectional view taken along line IX—IX of FIG. 8;

FIG. 10 is an enlarged view of a part of a conventional injection moldedresin gear, which shows a state that failure (sink mark or shrink mark)is caused in the gear; and

FIG. 11 is a side view of a conventional injection molded resin gear,which is viewed from the side of a web.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the accompanying drawings, the preferred embodiments ofthe present invention will be described below in detail.

[First Preferred Embodiment]

FIGS. 1 and 2 show the first preferred embodiment of an injection moldedresin gear 1 according to the present invention, FIG. 1 being a sideview of the injection molded resin gear 1 and FIG. 2 being a sectionalview taken along line II—II of FIG. 1.

The injection molded resin gear 1 is formed by injecting a resinmaterial, such as polyacetal, polyamide, polybutylene terephthalate orpolycarbonate.

The injection molded resin gear 1 shown in these figures comprises asubstantially cylindrical hub (inner cylindrical portion) 4 having anaxial hole 2 into which a shaft 3 is fitted, a substantially disk-shapedweb (disk-shaped portion) 5 formed on the outer peripheral side of thehub 4, and a substantially cylindrical rim (outer cylindrical portion) 6formed on the outer peripheral side of the web 5. The injection moldedresin gear 1 is formed with teeth 7 on the outer periphery of the rim 6(see FIG. 3). The web 5 is arranged in a substantially central portionin face width directions (see FIG. 2). The injection molded resin gear 1is substantially symmetric with respect to a plane including the centerof the web 5 (see FIG. 2).

On both sides 8, 8 of the web 5, a plurality of (first through third)annular circumferential ribs 10, 11 and 12 are formed concentricallywith the hub 4 and rim 6. The first circumferential rib 10 is arrangedso as to face the outer periphery of the hub 4 to be connected theretoby means of a plurality of first radial ribs 13 which extend radiallyfrom the outer periphery of the hub 4. The second circumferential rib 11is arranged so as to face the outer periphery of the firstcircumferential rib 10 apart therefrom. The third circumferential rib 12is arranged between the second circumferential rib 11 and the rim 6 tobe connected to the second circumferential rib 11 by means of aplurality of second radial ribs 14 which extend radially from the outerperiphery of the second circumferential rib 11.

The first radial ribs 13 are arranged on the outer periphery of the hub4 at regular intervals so as to protrude from the sides 8, 8 of the web5. The side shape of the first radial rib 13 is formed so that its widthgradually increases from the first circumferential rib 10 toward the hub4 (see FIG. 2). Thus, the root portion of the web 5 (the connectingportion of the web 5 to the hub 4) can be reinforced. The second radialribs 14 are arranged on the outer periphery of the secondcircumferential rib 11 at regular intervals so as to protrude from thesides 8, 8 of the web 5. The second radial ribs 14 have the same height(width) as that of the second and third circumferential ribs 11 and 12.

FIG. 4 is an enlarged view of a part of FIG. 2 for explaining therelationship between the thickness (t1) of the bottom 15 of the rim 6and the thickness (t2) of the web 5. As shown in this figure, theinjection molded resin gear 1 in this preferred embodiment is formed sothat the thickness (t1) of the bottom 15 of the rim 6 is smaller thanthe thickness (t2) of the web 5 (t1<t2). The injection molded resin gear1 is also formed so that the thickness of the bottom 15 of the rim 6 issubstantially constant (t1) in face width directions Y. In the injectionmolded resin gear 1 in this preferred embodiment, the thickness of eachpart (t1, t2, t3) is determined so that the thickness (t3) of the secondand third circumferential ribs 11 and 12 (see FIG. 1), the thickness(t1) of the bottom 15 of the rim 6, and the thickness (t2) of the web 5is t1≦t3<t2.

As shown in FIG. 1, the first circumferential rib 10 is arranged so asto face a plurality of injection molding pin point gates 16. The pinpoint gates 16 are arranged at regular intervals in circumferentialdirections of the first circumferential rib 10 so as to concentricallysurround the hub 4 similar to the first circumferential rib 10. Theoptimum value of the thickness (t4) of the first circumferential rib 10is suitably determined in accordance with the dimension of the tips ofthe pin point gates 16. The height of the first circumferential rib 10is set so that scars caused by cutting the pin point gates 16 are notobstacle without carrying out any after-treatments.

The optimum value of the thickness (t5) of the second radial ribs 14 isdetermined with respect to the thickness (t1) of the bottom 15 of therim 6 and the thickness (t2) of the web 5 so that t1≦t5<t2. Furthermore,if the thickness (t5) of the second radial ribs 14 is set with respectto the thickness (t3) of the second and third circumferential ribs 11and 12 so that t5<t3, the variation in flow of the molten resin can bemore remarkably reduced by the second and third circumferential ribs 11and 12, so that the flow of the molten resin can be more uniformlycaused.

Referring to FIG. 1, the relationship between the pin point gates 16 andthe first and second radial ribs 13 and 14 will be described below.

The first and second radial ribs 13 and 14 are displaced incircumferential directions from a straight line L1 which is drawn byconnecting the rotation center P of the injection molded resin gear 1and each of the pin point gates 16, i.e., a radial line L1 which is astraight line extending radially from the rotation center P, so as notto be arranged on the radial line L1. In addition, the first radial ribs13 and the second radial ribs 14 are displaced in circumferentialdirections from each other so as not to be arranged on the same radialline.

If the first radial ribs 13 are arranged on the radial line L1, there isthe possibility that the molten resin injected from the pin point gates16 rapidly flows along the first radial ribs 13 in radial directions(due to a so-called runner effect caused by the radial ribs), to makethe flow of the molten resin ununiform to deteriorate the roundness ofthe hub 4 to deteriorate the rotational accuracy of the injection moldedresin gear 1. However, according to this preferred embodiment, it ispossible to prevent such failure since the first radial ribs 13 are notarranged on the radial lines L1 passing through the pin point gates 16.

In addition, the second radial ribs 14 are not arranged on radial lines(weld lines) which are formed by the joining of the molten resin flowingout of adjacent two of the pin point gates 16. That is, the secondradial ribs 14 are not arranged on the radial lines L1 and weld lines,in which the flow of the molten resin is easy to be ununiform, so thatthe ununiformity of the flow of the molten is not promoted.

According to the injection molded resin gear 1 with this construction,if the molten resin is injected into the cavity from the plurality ofpin point gates 16 arranged at regular intervals in circumferentialdirections, the molten resin flows from the injected positions in radialdirections. Most of the injected molten resin flows along the firstcircumferential rib 10 in circumferential directions, and part of theinjected molten resin gradually flows in radial directions. Then, themolten resin flowing along the first circumferential rib 10 incircumferential directions meets the molten resin, which is injectedfrom the adjacent pin point gates 16, to flow in radial directions.

The molten resin flowing from the first circumferential rib 10 outwardlyin radial directions flows into the second circumferential rib 11 whichis perpendicular to the direction of the flow of the molten resin and inwhich the cross-sectional area of the passage for the molten resinrapidly increases, so that the variation in flow of the molten resin canbe suppressed by the second circumferential rib 11 even if the strengthof the flow of the molten resin outwardly in radial directions slightlyvaries. Moreover, the molten resin flowing outwardly in radialdirections from the second circumferential rib 11 passes through the web5 and second radial ribs 14 to flow into the third circumferential rib12 which is perpendicular to the direction of the flow of the moltenresin and in which the cross-sectional area of the passage for themolten resin rapidly increases, so that the variation in flow of themolten resin can be suppressed by the third circumferential rib 12 evenif the strength of the flow of the molten resin in radial directionsfrom the second circumferential rib 11 to the third circumferential rib12 slightly varies. As a result, the molten resin substantiallyuniformly flows inwardly and outwardly in radial directions from thefirst circumferential rib 10 to substantially make the cooling rate ofthe molten resin uniform to make the rate of shrinkage of the moltenresin uniform in circumferential directions of the web 5, so that it ispossible to form an injection molded resin gear 1 having a goodroundness.

According to this preferred embodiment, the thickness (t1) of the bottom15 of the rim 16 is smaller than the thickness (t2) of the web 5, sothat the shrinkage of the rim 6 having the teeth 7 can be decreased. Inaddition, the second and third circumferential ribs 11 and 12 which arethinner than the web 5 are solidified prior to the solidification of theweb 5, and then, the webs is solidified, so that the deformation of theweb 5 due to the decrease of the diameter thereof can be suppressed bythe second and third circumferential ribs 11 and 12, which aresolidified prior to the solidification of the web 5, due to the timedifference in solidification of the molten resin. As a result, theshrink mark in the connecting portion of the web 5 to the rim 6, i.e. inthe substantially central portion in face width directions of the teeth7, can be particularly decreased, so that shape error, such as totalalignment error, can be decreased to more precisely form the injectionmolded resin gear 1.

According to this preferred embodiment, the first circumferential rib 10is connected to the hub 4 by the first radial ribs 13, and the secondcircumferential rib 11 is connected to the third circumferential rib 12by the second radial ribs 14. Therefore, the rigidity of the web 5 canbe enhanced to suppress the deformation of the web 5 due to load duringpower transmission, so that it is possible to precisely transmitrotation.

According to this preferred embodiment, no radial ribs are not formedbetween the first circumferential rib 10, on which the pin point gate 16are arranged, and the second circumferential rib 11 which are arrangedradially outward from the first circumferential rib 10 so as to beadjacent thereto. Therefore, the first circumferential rib 10 functionsas a ring gate, so that the balance of the filling of the molten resincan be improved.

As described above, according to this preferred embodiment, it ispossible to obtain a precise injection molded resin gear 1 by thesynergistic effect of the above described effects. Therefore, a colorcopying machine or color printer using the injection molded resin gear 1in this preferred embodiment for transmitting power to a paper feedingcarrier belt or a photosensitive material can prevent print failure,such as color deviation, to enable a high quality color print. Inaddition, a power transmission mechanism using the injection moldedresin gear 1 in this preferred embodiment can precisely transmit powerof an actuator, such as a motor.

Furthermore, a single or a plurality of circumferential ribs may beformed between the third circumferential rib 12 and the rim 6 inaccordance with the outside diameter of the injection molded resin gear1. FIG. 5 shows an injection molded resin gear 1 wherein a fourthcircumferential rib 17 is formed between the third circumferential rib12 and the rim 6, the fourth circumferential rib 17 being not connectedto the third circumferential rib 12 and the rim 6 by radial ribs. Withthis construction, the rigidity of the web 5 can be enhanced, and themolten resin can more uniformly flow from the pin point gates 16 to therim 6.

FIGS. 6 and 7 show an example where the injection molded resin gear 1 inthis preferred embodiment is driven. As shown in these figures, each ofthe protrusions 20 of gear driving members 18, which are mounted so asto be rotatable with the shaft 3 fitted into the hub 4, engages a spacebetween adjacent two of the second radial ribs 14 to transmit therotation of the injection molded resin gear 1 to the shaft 3 via thegear driving members 18 or to transmit the rotation of the shaft 3 tothe injection molded resin gear 1 via the gear driving members 18. Inthis example, the engagement of the second radial ribs 14 with theprotrusions 20 of the gear driving members 18 prevents the relativerotation of the injection molded resin gear 1 to the shaft 3, and thesecond radial ribs 14 function as a detent.

While each of the protrusions 20 of the gear driving members 18 hasengaged the space between adjacent two of the second radial ribs 14 inthe example shown in FIGS. 6 and 7, the present invention should not belimited thereto, but each of the protrusions 20 of the gear drivingmember 18 may engage a space between adjacent two of the first radialribs 13. In this case, the first radial ribs 13 function as a detent.

The detent for the injection molded resin gear 1 should not be limitedto the first or second radial ribs 13 or 14, but a simple protrusion orrecessed portion being engageable with the protrusions 20 of the geardriving member 18 if it can prevent the relative rotation to the geardriving member 18.

[Second Preferred Embodiment]

FIGS. 8 and 9 show the second preferred embodiment of an injectionmolded resin gear according to the present invention. The injectionmolded resin gear 1 in this preferred embodiment has a plurality ofresin guiding protrusions 21, which are arranged at regular intervals soas to correspond to pin point gages 16, for guiding a molten resin,which is injected from the pin point gates 16, to a hub 4.

According to this preferred embodiment with such a construction, themolten resin injected from the pinpoint gates 16 substantially uniformlyflow in radial directions from the hub 4 to a rim 6. That is, since thehub 4 functions as a ring gate, weld lines (radial junction lines causedwhen the molten resins injected from adjacent two of the pin point gates16 meet each other between the adjacent two of the pin point gates 16)are difficult to be caused on the web 5, so that the flow of the moltenresin from the hub 4 toward the rim 6 is uniform in circumferentialdirections of the web 5.

The injection molded resin gear 1 in this preferred embodiment has thesame construction as that in the first preferred embodiment, except forthe position of the pin point gates 16. Therefore, the injection moldedresin gear 1 in this preferred embodiment can obtain the sameadvantageous effects as those in the first preferred embodiment.

[Other Preferred Embodiments]

The present invention can not only be applied to the injection moldedresin gear 1 in the above described preferred embodiments, but theinvention may also be widely applied to injection molded resin rotatingbodies, such as an injection molded resin sprocket wherein teethengageable with a chain are formed on the outer periphery of a rim, andan injection molded resin pulley wherein teeth engageable with a toothedbelt are formed on the outer periphery of a rim. In the injection moldedresin sprocket, the thickness of the bottom of the rim corresponds tothe thickness (t1) of the bottom 15 of the injection molded resin gear1. In the injection molded resin pulley, the thickness of the bottom orthe thickness of the core engaging the belt corresponds to the thickness(t1) of the bottom 15 of the injection molded gear 1.

Moreover, the present invention may be applied to a shaft supportingmember (an injection molded article) which is injection-molted so thatan outer cylindrical portion is connected to an inner cylindricalportion, which is concentrically therewith, by a disk-shaped member forsupporting a sliding or rotating shaft on the inner cylindrical portion.The present invention may also be applied to an injection molded resinroller (an injection molded article) wherein an inner cylindricalportion, into which a shaft is fitted, is connected to an outercylindrical portion, which rolls on another member, by a disk-shapedportion. In these injection molded articles, the thickness of the outercylindrical portion is smaller than the thickness of the disk-shapedportion in the vicinity of the connecting portion of the outercylindrical portion to the disk-shaped portion. Thus, in the injectionmolded articles, the shrink mark in the connecting portion of the outercylindrical portion to the disk-shaped portion can be decreased, so thatthe precision of the shape can be enhanced.

According to the present invention, at least one of the circumferentialribs 10, 11 and 12 and radial ribs 13 and 14 (all of the ribs 10 through14 if necessary) may be omitted in view of the outside diameter of theinjection molded resin gear 1.

In the injection molded resin gear 1 in the above described first andsecond preferred embodiments, the web 5 has been positioned in thesubstantially central portion in face width directions. However, thepresent invention should not be limited thereto, but the web 5 may beformed so as to be arranged on one side in face width directions.

In the above described preferred embodiments, if the thickness of therim varies in face width directions, the thickness (t1) of the bottom 15is smaller than the thickness (t2) of the web 5 in the vicinity of theconnecting portion of the rim 6 to the web 5.

As described above, according to the present invention, the thickness ofthe bottom of the rim outside of the web is smaller than the thicknessof the web, so that the deformation of the connecting portion of the rimto the web due to the decrease of the diameter thereof can be suppressedto inhibit the sink mark from being caused in the connecting portion ofthe rim to the web, thereby further improving the precision of the gear.

In addition, according to the present invention, the thickness of theouter cylindrical portion is smaller than the thickness of thedisk-shaped portion, so that the deformation of the connecting portionof the outer cylindrical portion to the disk-shaped portion due to thedecrease of the diameter thereof can be suppressed to inhibit the sinkmark from being caused in the connecting portion of the outercylindrical portion to the disk-shaped portion, thereby improving theprecision of the shape of the injection molded article.

While the present invention has been disclosed in terms of the preferredembodiment in order to facilitate better understanding thereof, itshould be appreciated that the invention can be embodied in various wayswithout departing from the principle of the invention. Therefore, theinvention should be understood to include all possible embodiments andmodification to the shown embodiments which can be embodied withoutdeparting from the principle of the invention as set forth in theappended claims.

1. An injection molded resin gear comprising: a substantiallycylindrical rim having teeth on an outer periphery thereof, said rimhaving a thickness (t1) in radial directions; a hub for receivingtherein a shaft; a web connecting said rim to said hub, said web havinga thickness (t2) in face width directions; and an annularcircumferential rib formed on said web between said rim and said hub soas to protrude from said web concentrically with said rim and hub, saidcircumferential rib having a thickness (t3) in radial directions,wherein a relationship between the thickness (t1) of said rim, thethickness (t2) of said web and the thickness (t3) of the circumferentialrib is t1≦t3<t2.
 2. An injection molded resin gear as set forth in claim1, wherein the thickness (t1) of said rim is smaller than the thickness(t2) of said web in the vicinity of a connecting portion of said rim tosaid web.
 3. An injection molded resin gear as set forth in claim 1,wherein said annular circumferential rib is formed on said web nearer tosaid hub than said rim, and wherein said injection molded resin gear isformed by injecting a molten resin from a plurality of pin point gateswhich are arranged at regular intervals in circumferential directions ofsaid circumferential rib.
 4. An injection molded resin gear as set forthin claim 1, which further comprises a resin guiding protrusion forguiding a molten resin, which is injected from a pin point gate, to saidhub, said resin guiding protrusion protruding from an outer periphery ofsaid hub on one side of said web so as to correspond to said pin pointgate.
 5. An injection molded resin gear comprising: a substantiallycylindrical rim having teeth on an outer periphery thereof, said rimhaving a thickness (t1) in radial directions; a hub for receivingtherein a shaft; a web connecting said rim to said hub, said web havinga thickness (t2) in face width directions; and a plurality of annularcircumferential ribs formed on said web between said rim and said hub soas to protrude from said web concentrically with said rim and hub, atleast one of said plurality of circumferential ribs having a thickness(t3) in radial directions, at least adjacent two of said plurality ofcircumferential ribs being connected to each other by a plurality ofradial ribs extending in radial directions, wherein a relationshipbetween the thickness (t1) of said rim, the thickness (t2) of the weband the thickness (t3) of said at least one of said plurality ofcircumferential ribs is t1≦t3<t2.
 6. An injection molded resin gear asset forth in claim 1, which further comprises a detent formed on saidweb, said detent engaging a gear driving member, which rotates with saidshaft, to allow said injection molded resin gear to rotate with saidgear driving member.
 7. An injection molded resin gear comprising: asubstantially cylindrical rim; a hub; and a web connecting said rim tosaid hub, said web having a plurality of annular circumferential ribswhich surround said hub concentrically with each other, at leastadjacent two of said circumferential ribs being connected to each otherin radial directions by means of a plurality of radial ribs which arearranged at regular intervals in circumferential directions, whereinsaid injection molded resin gear is formed by injecting a molten resinfrom a plurality of pin point gates arranged at regular intervals on acircle, which is arranged concentrically with said hub, nearer to saidhub than said adjacent two of said circumferential ribs, said radialribs being formed so as to be displaced from a straight line whichradially extends from a rotation center of said gear via each of saidpin point gates and so as to be displaced from a straight line whichpasses through the rotation center of said gear and a center betweenadjacent two of said pin point gates, and each of said radial ribsextending outwardly from one of said circumferential ribs, which isnearest to said pin point gates, is displaced from said pin point gatesin circumferential directions.
 8. An injection molded resin rotatingbody comprising: a substantially cylindrical rim having a thickness (t1)in radial directions; a hub for receiving therein a shaft; a webconnecting said rim to said hub, said web having a thickness (t2) inface width directions; and an annular circumferential rib formed on saidweb between said rim and said hub so as to protrude from said webconcentrically with said rim and hub, said circumferential rib having athickness (t3) in radial directions, wherein a relationship between thethickness (t1) of said rim, the thickness (t2) of said web and thethickness (t3) of the circumferential rib is t1≦t3<t2.
 9. An injectionmolded article comprising: an outer cylindrical portion having athickness (t1) in radial directions; an inner cylindrical portion forreceiving therein a shaft; a disk-shaped portion connecting said outercylindrical portion to said inner cylindrical portion, said disk-shapedportion having a thickness (t2) in face width directions; and an annularcircumferential rib formed on said disk-shaped portion between saidouter cylindrical portion and said inner cylindrical portion so as toprotrude from said disk-shaped portion concentrically with said outerand inner cylindrical portions, said circumferential rib having athickness (t3) in radial directions, wherein a relationship between thethickness (t1) of said outer cylindrical portion, the thickness (t2) ofsaid disk-shaped portion and the thickness (t3) of said circumferentialrib is t1≦t3<t2.